Astrophile: Elderly couple kicked out of the galaxy

Astrophile is our weekly column on curious cosmic objects, from the solar system to the far reaches of the multiverse

Object: A pair of white dwarfsStatus: Bounced from the galaxy

The two old stars were a happy couple. Childhood sweethearts, they continued to orbit each other even in retirement, and they lived peacefully in the same neighbourhood where they were born some 10 billion years earlier.

Then the unthinkable happened. A mid-sized black hole appeared in their star cluster, and a chance encounter with the bully sent the couple running – away from the cluster and on a path out of our galaxy entirely.

"We don't exactly know where it came from," says Mukremin Kilic, an astronomer at the University of Oklahoma in Norman, although the most likely origin is an old globular star cluster – a tightly packed grouping of stars that can harbour an intermediate-mass black hole at its core.

Third companion

Kilic says it is likely the binary pair once had a third companion orbiting them. When the trio swung too close to a large black hole, the third star was gobbled up and lost a large amount of energy as it fell in. That energy was transferred to the pair of white dwarfs, causing them to shoot out of the cluster.

Astronomers used to think that the hyper-speed duo, named LP 400-22, was a single star 1400 light years away. Later work showed that the star is really a tightly bound pair, and the newest measurements reveal that they are at least 2700 light years from us.

"That was completely unexpected," says Kilic. The greater distance is important, because astronomers calculate a star's velocity in part from its apparent movement across the sky year after year. The further away a star is, the greater its speed for a given movement across our line of sight. Based on the latest readings, the white dwarfs are moving at more than 830 kilometres per second – fast enough to escape the gravity of the Milky Way.

Further insight should come from Gaia, a soon-to-be-launched spacecraft that will measure distances to a billion stars in our galaxy, including this pair. "With that measurement, we can hopefully pinpoint where it came from," says Kilic.

Sun facing eviction?

Understanding the origins and behaviour of such high-speed stars can help astronomers trace the distribution of mass in the Milky Way. Some runaway stars, for instance, are already racing beyond the edge of our galaxy's visible disk and probing the dark matter halo, a cloud of the invisible stuff that envelops the Milky Way. Astronomers can use the motions of these stars to help reveal the amount of dark matter in the halo. Others may even prove useful filling in our map of the parts of the galaxy we can't see.

Is there any danger our sun will get ejected from the galaxy? We are safe for now, a single star far from any mid-sized black holes. But billions of years from now, the Milky Way will smash into our large galactic neighbour Andromeda. That collision will shuffle stars every which way, and it might even catapult our ageing solar system out of the Milky Way's embrace.

If you would like to reuse any content from New Scientist, either in print or online, please contact the syndication department first for permission. New Scientist does not own rights to photos, but there are a variety of licensing options available for use of articles and graphics we own the copyright to.

Runaway stars like this tend to be young loners that have come off worst in a clash with a more massive neighbour (Image: NASA/JPL-Caltech/UCLA)